In order to drive a vehicle in comfort, it is important that the air pressure in the tires is adjusted. This is because the problem of degradation in ride quality and fuel consumption might occur if the air pressure drops below an appropriate value. Therefore, a system for automatically detecting low-pressure in the tires (TPMS: Tire Pressure Monitoring System) has been researched. Information indicating that the tires have low pressure can be used to warn the driver, for example.
Methods for detecting low-pressure in the tires include methods for indirectly evaluating low-pressure in the tires by using alternative index values as well as methods for directly measuring the air pressure in the tires by, for example, attaching a pressure sensor to the tires. A dynamic loaded radius (DLR) method and a resonance frequency method (RFM) are known as examples of such indirect evaluation methods. The DLR method takes advantage of the phenomenon in which a low-pressure tire has a reduced dynamic loaded radius due to being deformed while travelling, and accordingly has an increased rotation speed. With the DLR method, low pressure in a tire is estimated from the rotation speed of the tire. The RFM takes advantage of the phenomenon in which the frequency characteristics of the wheel speed vary due to a pressure decrease.
JP 2011-247645A (hereinafter referred to as Patent Literature 1) discloses a detection device employing the DLR method, and JP 2008-110742A (hereinafter referred to as Patent Literature 2) discloses a detection device employing a combination of the DLR method and the resonance frequency method. Patent Literatures 1 and 2 refer to three index values, namely DEL1 to DEL3, which are low-pressure index values for evaluating low-pressure in the case of the DLR method. In Patent Literature 2, DEL1 to DEL3 are defined as follows. Note that V1, V2, V3, and V4 respectively denote the wheel speed of the front left tire, the wheel speed of the front right tire, the wheel speed of the rear left tire, and the wheel speed of the rear right tire.DEL1=[(V1+V4)/2−(V2+V3)/2]/[(V1+V2+V3+V4)/4]×100(%)DEL2=[(V1+V2)/2−(V3+V4)/2]/[(V1+V2+V3+V4)/4]×100(%)DEL3=[(V1+V3)/2−(V2+V4)/2]/[(V1+V2+V3+V4)/4]×100(%)